1. Field of the Invention
The present invention relates to a packet multiplexer for multiplexing low speed channels into a high speed channel in a communication system.
2. Description of the Background Art
In order to utilize a transmission path economically, a multiplexer is used in assembling several low speed channels and connecting them to a high speed channel. As a method for multiplexing, a time division multiplexing and a frequency division multiplexing are available, but here a conventional time division multiplexing scheme will be described.
As a conventional time division multiplexer, a time division multiplexer (TDM), a high functional channel division multiplexer (S-TDM), etc. have been used. TDM is a multiplexer in which time-slots are fixedly allocated with respect to a plurality of low speed channels and multiplexed on a time axis, while S-TDM is a multiplexer in which an improvement of a data transmission efficiency is realized by efficiently allocating vacant time-slots when there is no information. Note that high speed signals transmitted in time-slot forms by a multiplexer at one end of a plurality of low speed channels will be converted into signals for low speed channels so as to recover original data by a multiplexer at another end (also called a demultiplexer: usually one multiplexer has both functions of the multiplexer and the demultiplexer) which carries out the similar operation in an opposite way.
Thus the conventional multiplexers have an object of simply bundling a plurality of low speed channels. Usually these low speed channels comprise a plurality of user planes (U-plane) for carrying out user data transfer between end-to-end users, and a control plane (C-plane) to be used for set up, maintenance, and release of calls, U-planes and connections between a user and a network. For example, in the Integrated Service Digital Network (ISDN), the B-channel plays a role of U-plane and the D-channel plays a role of C-plane.
Now, in the conventional multiplexer (MUX) shown in FIG. 1, suppose that telephone numbers No. 1 to No. 23 are respectively allocated by the dial-in service to 23 channels accommodated in the MUX. In FIG. 1, an encircled number represents telephone numbers (only No. 1 to No. 3 are depicted). Then in the case where a person at No. 1 makes a telephone call to a person at No. 3, the communications become possible as a U-plane (B-ch) is established through a switching system by the function of a C-plane (D-ch), similarly as in the case of making a telephone call to a person at No. 2. In this case, it is convenient in that a telephone number is fixed to each channel, but despite of the fact that channels of No. 1 and No. 3 are accommodated in the same MUX, the U-plane is to be established via a demultiplexer (DE-MUX) and a switching system which are connected beyond the MUX and then switched back to the same MUX again, so that a wasteful route occurs in the established U-plane and the effective utilization of communication facilities is not realized.
Next consider the case where a PBX (Private Branch eXchanger) is introduced instead of MUX as shown in FIG. 2. In this case, when a person at No. 1 makes a telephone call to No. 3, a communication path is short-cut by the function of PBX and established without going up to a switching system so that it is possible to realize the optimization of a communication path. On the other hand, when a person at No. 1 makes a telephone call to No. 2, it is necessary for that person to explicitly specify the use of an external line by carrying out the so called 0 dialing. Even in this case, if a person at No. 1 makes a telephone call to a person at No. 3 after carrying out the 0 dialing, the resulting communication path will be one via a switching system. Thus, conventionally, the optimization of a communication path has been realized by requiring a user to explicitly specify for it.